Question

Which of the following is unchanged at the end of the CNO cycle?

A. carbon-12

B. oxygen-15

C. nitrogen-13

D. the four protons

Answer 1

Answer:

The correct option is: A. carbon-12

Explanation:

The CNO cycle, the abbreviation for the carbon-nitrogen-oxygen cycle, is a catalytic cycle by which the stars produce helium from elemental hydrogen, via a series of nuclear fusion reactions.

This cycle involves the fusion of four protons with carbon ($_{6}^{12}\textrm{C}$), nitrogen isotope ($_{7}^{13}\textrm{N}$), and oxygen isotope ($_{8}^{15}\textrm{O}$), to give an alpha particle and two electron neutrinos and positrons.

The reaction involves the regeneration of carbon ($_{6}^{12}\textrm{C}$) nucleus in the last step.

$_{6}^{12}\textrm{C}$ → $_{7}^{13}\textrm{N}$ → $_{6}^{13}\textrm{C}$ → $_{7}^{14}\textrm{N}$ → $_{8}^{15}\textrm{O}$ → $_{7}^{15}\textrm{N}$ → $_{6}^{12}\textrm{C}$